                Patent document 1: JP-H7-507513 A (corresponding to U.S. Pat. No. 5,602,524)        Patent document 2: JP-2007-15491 A (corresponding to US-2007/0008097 A1)        
A conventional tire air pressure detection apparatus includes a direct type, Herein, a transceiver contains a pressure sensor or the like to detect a tire air pressure. The transceiver is directly arranged in each of wheels attached to tires of a vehicle. Further, the body of the vehicle is equipped with an antenna and a receiver to receive a detection signal from the transceiver and detect a tire air pressure.
Such a direct-type tire air pressure detection apparatus uses ID information, which is assigned to a corresponding wheel to thereby determine whether the received data is transmitted inside of the subject vehicle and which wheel the data transmitting transceiver is attached to.
Such ID information is previously registered in the receiver. When receiving the data sent from the transceiver, the corresponding wheel can be determined based on the received ID information. In detail, the receiver is set to a transceiver ID registration mode; thereby, the trigger unit transmits a trigger signal to the transceiver. The transceiver transmits, in synchronization, data towards the receiver to thereby perform registering the ID (see Patent document 1).
Herein, it is assumed that a user changes the positions of the wheels, for instance, for rotating tires inside the relevant vehicle. In such a case, the tire air pressure detection apparatus cannot respond to the wheel position change. The user thus needs to read ID information of the wheels having been rotated and re-register ID information of the wheels. This poses complicated re-registration work, worsening the efficiency in rotating the tires.
Accordingly, it is desirable to detect a wheel position to which a transceiver is attached even without assigning ID information. Alternatively, it is desirable that the ID information can be automatically re-registered after the position change of the wheels.
Patent document 2 proposes a new technology, in which a trigger unit transmits a trigger signal having a constant strength; and each transceiver attached to each wheel detects a reception strength of the received trigger signal. Herein, it is considered that the strength of the trigger signal declines as the distance from the trigger unit increases; thus, the wheel position is detected based on the reception strength detected by the corresponding transceiver.
Further, to detect the wheel position appropriately, two trigger units of a front-side trigger unit and a rear-side trigger unit are arranged individually to correspond to the front wheels and the rear wheels, respectively.
In Patent document 2, for instance, the front-side trigger unit is arranged in a position different from the positions of the front wheels; while the rear-side trigger unit is arranged in a position different from the positions of the rear wheels. In such a case, one wheel may be closer to the corresponding trigger unit than the other wheel. Thereby, it should be designed that the transceiver attached to the farther-side wheel can receive a trigger signal outputted from the trigger unit and this should be achieved in both the front side and the rear side of the vehicle.
To achieve the above, the strength of the trigger signal may be increased. However, the signal outputted from the trigger unit for the front wheels may be received by the transceiver attached to the rear wheel. In other words, the trigger signal may be received by a transceiver other than a transceiver which should receive the trigger signal. In addition, the strength of the trigger signal may be restricted by regulations etc. The strength may be controlled at a certain level.
In contrast, there may be a cause for an attenuation of the trigger signal, such as a significant distance to the farther-side transceiver or an intervening object or metal member in an intermediate position to the farther-side transceiver. When the strength of the trigger signal is small, the field strength spread to each transceiver may become too low to keep the communication state stable.